The present invention relates to a method for detecting the oscillation criticality of a servo control system and adjusting control parameters.
Usually, in order to control an object to be controlled in a servo control system, feedback control is carried out, by which an amount of operation with respect to the object to be controlled is acquired from a deviation between a command issued from an upper-level apparatus and an actual amount of control. FIG. 5 is a block diagram showing a construction of a servo control system by which speed control is carried out. The servo control system is composed of a subtractor 1, a speed controller 2, a torque amplifier 3, a servo motor (M) 4, an encoder (E) 5, a machine 6, and a differentiator 7. The speed controller 2 is a means for controlling the machine 6, which is an object to be controlled, and is a proportion-integration-differentiation unit (hereinafter called a xe2x80x9cPID controllerxe2x80x9d). Herein, Kv, Ki and Kd are control parameters of the speed controller 2. Kv is a proportional gain, Ki is a reciprocal number of an integration time constant, and Kd is a differentiation time.
The subtractor 1 subtracts a speed feedback xcfx89 from a speed command xcfx89r inputted from an upper-level apparatus (not illustrated), and outputs a speed deviation. The speed controller 2 carries out PID control by inputting the speed deviation and outputs a torque command Tr. The torque amplifier 3 outputs a current to the servomotor 4 by inputting the torque command Tr. The servomotor 4 rotates by the current and the machine 6 moves by the rotation movement. The encoder 5 is attached to the servomotor 4 and outputs the rotation position of the servomotor 4. The differentiator 7 differentiates the rotation position, which is outputted from the encoder 5, and outputs a speed feedback xcfx89. Also, in the case where the above-described servo control system is a digital control system that carries out control per sampling cycle, usually, there are many cases where, using a difference detector instead of the differentiator 7, a difference between the previous rotation position and this rotation position is made into the speed feedback xcfx89.
FIG. 6 is an equivalence block diagram of the servo control system shown in FIG. 5. In FIG. 6, a description is based on the assumption that the machine 6 is completely rigid, the response of the torque amplifier 3 is ideal for simplifying the description thereof, and the speed controller 2 carries out proportional control on the basis of only the proportional gain Kv. FIG. 6(a) is an equivalence block diagram of a servo control system where an inertia of the machine 6 is assumed to be J, and FIG. 6(b) is an equivalence block diagram of a servo control system where an inertia of the machine 6 is assumed to be 2J. Herein, it is also assumed that values of the proportional gains Kv in FIGS. 6(a) and (b) are the same.
FIG. 7 is a graph showing a transition response of the speed feedback xcfx89 with respect to a step-like speed command xcfx89r in FIGS. 6(a) and (b). As shown in FIG. 7, where the inertia of the machine 6 is changed from J to 2J, it is understood that the response of the servo control system changes and the followability of the servo control system is worsened.
Therefore, in such a servo control system, in a case where parameters of an object to be controlled such as the inertia of the machine 6 is changed, it is necessary to vary the control parameters such as the proportional gain Kv of the speed controller 2 in response to the value of the inertia so that the machine 6 is optimally controlled. However, if the control parameters such as the proportional gain Kv are thoughtlessly changed, there may be a cause a concern that oscillations occur due to resonance of the mechanical system including the machine 6 and useless time of the servo control system, etc. Generally, the larger the proportional gain Kv becomes, the greater the followability to the speed command xcfx89r is increased. But, if the proportional gain Kv is increased too much, the servo control system is likely to oscillate.
It is assumed that, among the values of the proportional gains Kv, an area of values of the proportional gain Kv is defined to be Area xe2x80x9caxe2x80x9d when the servo control system does not oscillate and is in a stable state, an area of values of the proportional gain Kv is defined to be Area xe2x80x9cbxe2x80x9d when the servo control system is in oscillation criticality, and an area of values of the proportional gain Kv is defined to be Area xe2x80x9ccxe2x80x9d when the servo control system is in a completely oscillating state. FIG. 8 are graphs showing a frequency response G(f) of the speed feedback xcfx89 in terms of a logarithm when the values of the proportional gain Kv are in respective areas.
FIG. 8(a) shows a state of log G(f) where the values of the proportional gain Kv are in Area xe2x80x9caxe2x80x9d, wherein log G(f) has a small peak in the vicinity of f=0, and the value of log G(f) is totally low. FIG. 8(b) shows a state of log G(f) where the values of the proportional gain Kv are in Area xe2x80x9cbxe2x80x9d, wherein, although log G(f) is distributed in a wide frequency band, the peak thereof is not so high. FIG. 8(c) shows a state of a frequency response log G(f) where the values of the proportional gain Kv are in Area xe2x80x9ccxe2x80x9d, wherein log G(f) has a very high peak at a certain frequency band. It is understood that the servo control system oscillates in this frequency band. In addition, the frequency response of the torque command Tr shows a tendency similar to the frequency response of the above-described speed feedback xcfx89.
As described above, the control parameters such as the proportional gain Kv causes the followability of the servo control system to be worsened if the values thereof are small, and brings about oscillations in the servo control system if the values thereof are large. Therefore, it is recommended that the control parameters such as the proportional gain Kv are set to optimal values.
As a method for optimally obtaining control parameters such as the proportional gain Kv, etc., such a method is disclosed by Japanese Patent Publication No. 2861394, in which an amplitude and a frequency of fluctuations of the speed feedback xcfx89 in an appointed duration of time are calculated, and the control parameters are adjusted by judging that, if the amplitude value and frequency value exceed appointed values, oscillations have occurred. However, with the method disclosed by the above-described patent publication, the control parameters cannot be adjusted unless actual oscillation occurs. For this reason, where this method is used, actual oscillation occurs before commencing to adjust the control parameters, whereby such problems are brought about, by which the machine 6 connected to the servomotor 4 may be damaged due to influences of the oscillation, or large noise may be generated.
On the other hand, it is experimentally made clear that shaking of the speed feedback xcfx89 and torque command Tr changes in response to fluctuations in the proportional gain Kv. The shaking of the speed feedback xcfx89 and torque command Tr means unevenness of frequency components in oscillations of the speed feedback xcfx89 and torque command Tr. FIG. 9 is a graph showing the relationship between the proportional gain Kv and the shaking amount of the speed feedback xcfx89. Where the value of the proportional gain Kv is in Area xe2x80x9caxe2x80x9d, the shaking amount of the speed feedback xcfx89 is small. And where the value of the proportional gain Kv is in Area xe2x80x9cbxe2x80x9d, the shaking amount of the speed feedback xcfx89 is gradually increased in line with an increase in the value of the proportional gain Kv. In Area xe2x80x9ccxe2x80x9d, that is, the oscillation area, although the speed feedback xcfx89 consistently oscillates, the frequency components of the oscillations are made almost constant, wherein the shaking amount is made small. Such a tendency is also brought about with respect to the shaking amount of the torque command Tr.
As described above, conventionally, when adjusting the control parameters in the servo control system, the amplitude and frequency of fluctuations of the speed feedback in an appointed duration of time are calculated, wherein if the amplitude and frequency exceed appointed values, it is judged that the servo control system oscillates, and the control parameters are adjusted. However, with this method, the control parameters cannot be adjusted unless actual oscillations begin. Accordingly, where the method is used, actual oscillations occur before commencing to adjust the control parameters, wherein there occur such problems, by which a machine connected to the servomotor is damaged due to influences of the oscillations, and large noise is generated.
It is therefore an object of the present invention to provide a method for detecting oscillation criticality of a servo control system that can adjust control parameters without causing the servo control system to oscillate.
In order to solve the above-described problem, the present invention is a method for detecting oscillation criticality of a servo control system, which can adjust control parameters of means for controlling an object to be controlled, by carrying out, per sampling cycle, input of a deviation between the amount of control fed back from a servomotor to drive the above-described object to be controlled and a command value inputted from an upper-level apparatus, and output of a torque command to a torque amplifier that outputs a current to the servomotor, comprising the steps of:
stepwise changing values of control parameters, and measuring a shaking amount being uneven in a frequency component of oscillations of the above-described variable per value of the control parameters;
judging that, when the above-described shaking amount exceeds an appointed value, the servo control system reaches oscillation criticality; and
adjusting the above-described control parameters by returning the values of the control parameters, which are set in the above-described controlling means, by only appointed steps.
The method for detecting oscillation criticality of a servo control system according to the invention can detect values of control parameters set when the servo control system reaches the oscillation criticality by detecting the shaking amount, that is, unevenness in a frequency component of oscillations of a variable that is maximized when the servo control system is in oscillation criticality, whereby the control parameters can be adjusted without causing the servo control system to oscillate.
As described above, the method for detecting oscillation criticality of a servo control system according to the invention can detect control parameters, when the servo control system is in oscillation criticality, by obtaining the shaking amount, that is, unevenness in frequency of oscillations of a torque command or speed feedback, which is maximized in the oscillation criticality area of the servo control system, whereby the control parameters can be adjusted without causing the servo control system to oscillate.